Wednesday, July 06, 2011

Photon Drag Image From Slow Light

This is a very fascinating experiment to show the photon drag effect in a fast-moving medium. By passing light through ruby (a "slow light" medium), the image made by the light tilts differently based on the direction of rotation of the medium from clockwise to counterclockwise. Not only that, they also see an increase in the tilt when they ramp up the photon intensity. They attributed this affect to multiple absorption and re-emission in the medium.

To test this idea, Franke-Arnold's group shone a rectangular beam of green light into a cylinder made of ruby, which they spun at up to 30 revolutions per second. Ruby is a well known slow-light material, and transmits light with a group velocity of just a few tens of metres per second. By changing the rotation of the ruby from clockwise to counter-clockwise, the researchers could detect a rotation of the light rectangle by about one-third of a degree (see figure). What is more, when they ramped up the intensity of the light, the rectangle's rotation increased to about 10° – probably because the photons were then undergoing several absorption and re-emission cycles.